Radiation Effects on Blood Vessels

Abstract

To account for the effects of radiation on the blood vessels three main theories have been advanced: the metabolic, the neuromuscular, and the cellular. According to the metabolic theory the vascular effect of the rays is a secondary one, brought about through metabolic changes in the adjacent tissues. There is, however, some question as to the exact nature of the agent set free in the irradiated tissues, to which the effect on the blood vessels is attributable (Wetterer, Schwarz, Lewis). Lewis, whose view is the most widely accepted, believes it to be of the nature of histamin (H-substance), and it is undoubtedly true that the vascular response to irradiation bears some resemblance to that induced by the cutaneous application of histamin and that mechanical and thermal irritants, as well as light, produce a comparable effect through the liberation of this substance. There are, nevertheless, numerous objections to the acceptance of the histamin theory. First, there are a number of agents, particularly among the salts of heavy metals, quite unrelated to histamin, which are capable of producing erythema and other vascular phenomena. In the second place there is no reason to assume that the blood vessels are exempt from the rules governing the reaction of living tissues in general to radiation. Furthermore there are certain differences in the erythema following irradiation and that produced by histamin, especially its longer latent period, and its rhythmic or wave-like character. Also to be taken into consideration are the absence of the diffuse reflex erythema observed with histamin, the fact that roentgen and radium rays are capable of producing not only a dilatation of capillaries, which is followed by erythema, but also a narrowing of their lumina preceding the erythema, and the failure as yet to demonstrate any increase of histamin in the irradiated tissue such as has been shown to exist following ultraviolet irradiation (Ellinger). As to this last, it may well be that different mechanisms are involved in the erythema produced by visible and ultraviolet light and that due to x-rays and radium, the former being a photo-chemical and the latter a photo-electrical effect. According to the neuromuscular theory the changes observed in irradiated blood vessels are produced by the direct action of the rays. Under their influence certain elements of neuromuscular character may be responsible for constriction or dilatation of the vessels, but whether it is the nerves that are primarily affected or the muscle cells is a matter of dispute (Krogh and Vintrup, Ricker, Glauber, Muller, David and Gabriel). Either assumption is open to numerous objections both from the standpoint of radiobiology and anatomy and physiology. As to the radiosensitivity of nerves and muscles, it may be stated that doses sufficient to produce erythema and allied vascular phenomena have proved ineffective in these tissues.